Nonaqueous lithium ion secondary batteries employing nonaqueous electrolytes are utilized today as secondary batteries with high energy density. In a nonaqueous lithium ion secondary battery, transfer of lithium ions between a positive electrode and a negative electrode causes charges and discharges.
A lithium transition metal composite oxide having a layered structure such as lithium-cobalt oxide (LiCoO2) is employed as the positive electrode, a carbon material capable of storing and releasing lithium, for example, is employed as the negative electrode, and a nonaqueous electrolyte produced by dissolving lithium salt such as lithium tetrafluoroborate (LiBF4) or lithium hexafluorophosphate (LiPF6) in an organic solvent such as ethylene carbonate or diethyl carbonate is used in such a nonaqueous lithium ion secondary battery.
Such nonaqueous lithium ion secondary batteries have recently been used as power supplies for various types of mobile equipment. With increasing power consumption due to expansion in functionality of the mobile equipment, however, it is desired that nonaqueous lithium ion secondary batteries have higher energy density.
Increased capacity of positive electrode materials is required to obtain nonaqueous lithium ion secondary batteries with high energy density. In particular, expectations for layered compounds are rising. Studies on many kinds of lithium containing layered compounds have so far been made, resulting in development of materials such as LiCoO2, LiNiO2 and LiNi1/3Co1/3Mn1/3O2.
A method for synthesizing a lithium compound via a sodium compound has been studied as a new synthesis method of a lithium compound. According to the method, a lithium containing layered compound, which is difficult to be synthesized, can be easily obtained. In particular, ion exchange of sodium for lithium allows Na0.7CoO2 and NaCo1/2Mn1/2O2 to be utilized as a positive electrode active material for a lithium ion battery.
Therefore, studies have recently been made on an ion exchange method using a synthesis method of sodium containing layered compounds and chemical techniques.
[Patent Document 1] JP 2002-220231 A
[Nonpatent Document 1] Solid State Ionics 149 (2002) P. 39